The present invention relates to the field of solid state capacitors. The invention particularly relates to capacitors of the type in which a powder-formed valve action metal forms a highly porous anode body portion of a capacitor, an electrically insulating dielectric layer is formed though the porous structure of the anode body, and a conducting cathode layer is formed on the dielectric layer and which is then electrically connected to a cathode terminal, the anode body being electrically connected to an anode terminal.
U.S. Pat. No. 5,357,399 (Salisbury) describes a method for simultaneously manufacturing multiple such capacitors from a porous tantalum layer sintered to a tantalum substrate. The porous layer is machined to form the anode bodies of each capacitor. After processing a top plate (substrate lid) is bonded to a the top ends of the anode bodies. The substrate/anode bodies/lid sandwich is then diced to separate a plurality of individual capacitors in each of which the substrate material forms the anode terminal and the lid material forms the cathode terminal. PCT application CB99/03566 (AVX Ltd) concerns a modified version of the Salisbury method in which the volumetric efficiency of the capacitors produced is optimized by removing the need for a substrate lid as the cathode terminal of each capacitor, thereby increasing the specific capacitive volume.
The foregoing methods permit the manufacture of very small but highly efficient capacitors. However the continued pressure of electronic circuit board design towards miniaturization of components and ease of assembly of such boards maintains a continued need for capacitors of improved volumetric efficiency and reduced component windows (or footprint) on the circuit board.
The present invention seeks to provide improved capacitors and improved methods of manufacturing such capacitors.
According to one aspect of the present invention there is provided a method of manufacturing solid state capacitors comprising: providing an electrically conducting substrate having a plurality of apertures formed therethrough; forming plurality of porous bodies comprising valve action material on the substrate, a portion of each porous body being accommodated in an associated aperture; forming an electrically insulating layer over the free surface of the porous bodies; forming a conducting cathode layer over the electrically insulating layer applied to the porous bodies; and providing cathode termination means on an exposed underside surface of each coated porous body accommodated in the aperture, providing anode termination means on an underside surface of the substrate adjacent the aperture, the anode termination forming an electrical contact with the substrate material and the cathode termination forming an electrical contact with the cathode layer, and dividing the substrate into individual capacitor units, each comprising a porous body accommodated in an aperture-defining portion of the substrate. In this way a plurality of capacitors may be manufactured, which capacitors have anode and cathode terminals on a common face of the capacitor.
Preferably respective anode and cathode termination means on each capacitor have generally coplanar exposed contact surfaces so that the capacitor may stand with its underside on a flat surface with the contact surfaces on the flat surface.
The porous bodies may be formed from a pre-form layer of porous valve action material pressed onto a top surface of the substrate. The pre-form layer may be machined or cut to form the individual porous bodies over the aperture and substrate top surface. Other shaping processes may be used however.
In one aspect of the invention both the porous body free surface and exposed substrate surface are coated with the insulating layer and a portion of the insulating layer over the substrate is removed in order to allow an electrical contact with the substrate.
A surface portion of the substrate underside may be masked to prevent formation of a dielectric layer and cathode layer thereon.
Preferably a conducting coating is provided over the cathode layer on the porous body. The conducting coating may extend over a substantial proportion of the surface of the porous body.
In another aspect of the invention a conducting wick is formed in the porous body, which wick electrically connects conducting layers applied to different ends or sides of the porous body. This reduces the internal ESR of the capacitive body, improving device performance
According to yet another aspect of the invention there is provided a solid state capacitor comprising: an electrically conducting substrate member; a porous body comprising valve action material abutting a surface portion of the substrate; an electrically insulating layer forced over the free surface of the porous cathode; a conducting cathode layer formed over the electrically insulating layer applied to the cathode body free surface; and an anode terminal in electrical contact with a surface of the substrate member and a cathode terminal provided in electrical contact with a surface of the porous body, wherein the terminals are formed on a common surface of the capacitor.
The substrate member may form an exposed frame on an underside of the capacitor with an anode terminal formed thereon and the cathode terminal is formed on a portion of the porous body surface located within the frame.
The frame may be open at one end, thereby giving a generally horse shoe configuration to the substrate member.
The anode terminal body and the anode body may each be formed from a pre-form layer of porous valve action material that has been applied to the substrate. The pre-form may be applied by laying a green, unsintered mixture of valve action metal powder and binder/lubricant on the substrate. The green mixture may then be sintered to fuse the powder into a solid highly porous pre-form, the binder/lubricant being burnt off during sintering.
The pre-form layer may be machined to form the porous bodies. Typically longitudinal and lateral grinding cuts may be employed in order to produce a network of rectilinear porous bodies on the substrate, separated by xe2x96xa1streetsxe2x96xa1 corresponding to the path of the grinding cut. Naturally more complex shapes can be produced by conventional machining techniques, as required.
The insulating layer may be a dielectric layer of an oxide of the valve action metal, applied for example by conventional anodization techniques in order to build up gradually an oxide of the required thickness and integrity. In one example in which the valve action layer is tantalum, a layer of tantalum pentoxide is built up on the bodies.
The cathode layer may be applied by dipping of the anode and cathode bodies into a precursor solution of, for example manganese nitrate solution. The layer of manganese nitrate formed on the bodies may be heated to oxidise the nitrate to manganese dioxide. Repeated dipping steps may be necessary in order to build-up the optimum cathode layer.
It may be necessary to remove cathode and insulating layer material from those parts of the substrate which are to form the anode terminal so that an electrical connection to the valve-action material may be made. Removal of the layers may be by machining, for example grinding, or by etching, for entangle RF etching. In one example grinding cuts are made along a top surface of each anode, thereby exposing valve action material. The top surface may then be given a termination process to form the anode terminal.
A termination process is also carried out on a surface of the porous body, in which carbon and silver layers are formed on the conducting cathode layer of the top surface. These two conducting layers provide a terminal for electrical connection, by for example soldering, to an electrical or electronic circuit.
The spaces between the porous bodies on a substrate may be filled with an insulating material, for example a liquid plastics resin which solidifies to form a protective encapsulation of the bodies. Naturally the resin must leave the terminals exposed. This may require the use of masking over the terminal contacts.
The next step which must be carried out is separation of the or each capacitor from the bulk substrate. This may be achieved by machining by for example a grinding cut. If necessary a rigid backing support may be provided for the substrate so as to provide the necessary structural rigidity to permit cutting without damaging the capacitors. One example of a suitable material is glass which is adhered to the substrate by means of a suitable adhesive. UV sensitive adhesive tape may conveniently be used, release of the separated capacitors being effected by exposing the adhesive on the tap to UV energy.
Any valve action material suitable for use in the formation of a solid state capacitor may be used in the process. A preferred example is tantalum, but other metals such as niobium and valve-action non-metals may be used. Preferably the substrate is formed from the same material as the anode, so as to provide chemical compatibility and uniform expansion during heating (sintering).